About postfix-docker

This container runs the postfix mail server, and is a combination of the following two containers:

• https://github.com/jessfraz/dockerfiles/tree/master/postfix
• https://github.com/juanluisbaptiste/docker-postfix

It combines both into one, using the alpine base and logging from the first container; and the configuration properties from the second.

It allows to run an SMTP server in a docker container, with various options:

• Send the mails directly, optionally signing them with DKIM
• Optionally rewrite headers
• Optionally relay the mails to an upstream mail server, optionally with authentication
• For advanced users: any arbitrary Postfix configuration and shell command can be added

How to run it

The following env variable is mandatory to be passed to the container:

• SERVER_HOSTNAME: Server hostname for the Postfix container. Emails will appear to come from this domain.

This will start the mail server with the specified hostname on port 25 with a maximum of 1000 recipients per mail.

These two parameters can be changed with the following environment variables:

• SMTP_PORT: (Default value: 25) Port number of the upstream SMTP server to use. (If set to 465 for SMTPS, this will automatically trigger options smtp_tls_wrappermode = yes and smtp_tls_security_level = encrypt).
• RECIPIENT_LIMIT: (Default value: 1000) Maximum number of recipients per mail.

If you wish to enable header rewriting, the following environment variables can be used:

• SMTP_HEADER_TAG: When set, will add a header for tracking messages upstream. Helpful for spam filters. Will appear as RelayTag: ${SMTP_HEADER_TAG} in the email headers.
• REWRITE_DOMAIN: When set, will change the part behind the @ character in all from-adresses to the specified domain, this is handy when your upstream server requires mails to be sent from a specific domain only (do not add the @ character itself).
• REWRITE_HEADERS: When set, will apply the regular expression(s) to all headers of all passing mails, enter the full regular expression; use seperator \n if you wish to apply multiple regexes (be sure to escape $ with \$ when using backreferences since this is an environment variable!)

If you wish to sign mails with DKIM, the following optional environment variabled can be used:

• DKIM_DOMAIN: All mails from this domain will be signed with DKIM, other domains will be untouched
• DKIM_KEY: Location of the file in the container with the DKIM private key (this needs to be mounted, see below)
• DKIM_SELECTOR: The name of the DKIM selector for the mail signature

If you wish to use an upstream server to send the mails, the following environment variables can be used:

• SMTP_SERVER: Server address of the upstream SMTP server to use.
• SMTP_USERNAME: Username to authenticate with.
• SMTP_PASSWORD: Password of the SMTP user.

If you wish to use a custom queue directory, i.e. mounting this outside of the docker container:

• QUEUE_DIRECTORY: Path (relative to the container) of a directory to store the mail queue

Adding arbitrary configuration properties

If the configuration options in the environment variables above do not suit your needs, you can specify a shell script which you can mount in the container that changes the postfix configuration file as much as you like. There is a function in the runit file add_config_value that you can use fo this. Call it as follows:

add_config_value "key" "value"

For example:

add_config_value "smtp_use_tls" "yes"

To activate this option, mount a shell script in the container in a place of your liking, and specify the location of the script in the environment variable ADDITIONAL_CONFIG:

docker run -d --name postfix -p "25:25" \
       -e SERVER_HOSTNAME=helpdesk.mycompany.com \
       -e ADDITIONAL_CONFIG=/tmp/additional_config.sh \
       -v /path/to/addition_config.sh:/tmp/additional_config.sh \
       cedricroijakkers/postfix-docker:3.8.5

It will be loaded and fully executed at container boot time. You can use any command in there, but it is recommended to stick to add_config_value as this will automatically replace any existing configuration value with the same name in the postfix configuration file.

WARNING: This will execute the script as-is without checking any contents of it. Be sure to use it wisely!

Starting the container: some examples

Here are some examples to use the mail server in various use cases:

To configure the mail server to relay mails directly, without any special configuration:

docker run -d --name postfix -p "25:25" \
       -e SERVER_HOSTNAME=helpdesk.mycompany.com \
       cedricroijakkers/postfix-docker:3.8.5

To configure the mail server with an external queue directory:

Create a directory /home/postfix/queue on your host OS (make sure this is owned by UID:GID 102:102), and then start the container as follows:

docker run -d --name postfix -p "25:25" \
       -e SERVER_HOSTNAME=helpdesk.mycompany.com \
       -e QUEUE_DIRECTORY=/data/postfix \
       -v /home/postfix/queue/data/postfix: \
       cedricroijakkers/postfix-docker:3.8.5

To configure the mail server to relay mails directly, and sign them for domain helpdesk.mycompany.com with DKIM selector sel1 (be sure to add the required DNS records to avoid mails ending up in the SPAM folder!):

docker run -d --name postfix -p "25:25" \
       -e SERVER_HOSTNAME=helpdesk.mycompany.com \
       -e DKIM_DOMAIN=notification.vodafone.nl \
       -e DKIM_KEY=/var/db/dkim/helpdesk.mycompany.com.private \
       -e DKIM_SELECTOR=sel1 \
       -v /path/to/helpdesk.mycompany.com.private:/var/db/dkim/helpdesk.mycompany.com.private \
       cedricroijakkers/postfix-docker:3.8.5

To configure an upstream server without authentication:

docker run -d --name postfix -p "25:25" \ 
       -e SMTP_SERVER=smtp.bar.com \
       -e SERVER_HOSTNAME=helpdesk.mycompany.com \
       cedricroijakkers/postfix-docker:3.8.5

To configure an upstream server with authentication:

docker run -d --name postfix -p "25:25" \ 
       -e SMTP_SERVER=smtp.bar.com \
       -e SERVER_HOSTNAME=helpdesk.mycompany.com \
       -e SMTP_USERNAME=foo@bar.com \
       -e SMTP_PASSWORD=XXXXXXXX \
       cedricroijakkers/postfix-docker:3.8.5

Maintainer

This container is built and maintained by Cedric Roijakkers.